• Pittarello, Lidia
• Claeys, Philippe
• Ji, Gang
• Yamaguchi, Akira
• Debaille, Vinciane
• Schryvers, Dominique

Abstract

Introduction: The occurrence of ringwoodite as highpressure<br/>polymorph of shocked olivine within shock veins in meteorites<br/>is relatively common, e.g., [1]. In some cases, shocked<br/>olivine displays a complex structure, with high-Fe ringwoodite<br/>rimming low-Fe olivine and fine-grained lamellae of undefined<br/>phases occurring in the olivine core [2-4]. Similar features were<br/>observed in the sample A09584 and were further investigated by<br/>scanning electron microscopy, electron microprobe, Raman spectroscopy,<br/>and transmission electron microscopy.<br/><br/>Shock veins and olivine clasts: In the investigated sample,<br/>classified as L6 [5], shock veins are 1-2 mm wide blackish portions<br/>under transmitted light, generally localized along grain<br/>boundaries. The shock veins consist of clasts, mostly of olivine<br/>and pyroxene, suspended in a glassy matrix, partially crystallized<br/>in 10 µm microlites with olivine composition. Olivine clasts are<br/>rimmed by a 50 µm thick layer of ringwoodite, which has a higher<br/>Fe/Mg ratio than the unshocked olivine (UO). The core of these<br/>clasts contains a dense network of dark (in BSE-SEM images)<br/>lamellae and whitish domains, whose nature could not be determined<br/>other than with TEM.<br/><br/>TEM results: The ringwoodite rim consists of an aggregate<br/>of hypidiomorphic grains, which have an average size of 500 nm<br/>and exhibit internal features that resemble stacking faults. The<br/>clast core contains: (a) domains with nanocrystals of either olivine<br/>or wadsleyite, with strong shape preferred orientation and<br/>lower Fe/Mg ratio than the UO, and (b) veinlets of maximum 500<br/>nm in thickness, composed of equigranular nanocrystals of olivine,<br/>with higher Fe/Mg ratio than the ringwoodite and the UO<br/>and with random orientation. No amorphous material has been<br/>detected.<br/><br/>Discussion: Our observations are in agreement with the most<br/>accepted hypothesis for the formation of the ringwoodite rim,<br/>which is solid state transformation due to diffusion controlled<br/>growth under high temperature conditions [2-4, 6]. An alternative<br/>explanation is fractional crystallization from olivine melt under<br/>shock pressure conditions [7], but an intermediate layer of wadsleyite<br/>should have formed. However, a pressure-composition<br/>phase diagram calculated for an ambient temperature of 1600°C<br/>[8], might explain also the different Fe/Mg ratios in the coexisting<br/>olivine, wadsleyite and ringwoodite. The peak pressure in the<br/>compression stage corresponds to a "triple point", where ringwoodite<br/>and wadsleyite, with respectively high and low Fe/Mg<br/>compositions, formed from olivine. The following release wave<br/>triggered melting of the remaining olivine along veinlets. The<br/>melt, enriched in Fe, lately crystallized as olivine

Topics

• impedance spectroscopy
• amorphous
• grain
• scanning electron microscopy
• melt
• transmission electron microscopy
• random
• phase diagram
• Raman spectroscopy
• crystallization
• stacking fault
• lamellae